Combining two shaders, one with color map, one with a specularity map
You want to use a mix shader (or sometimes an add shader) to combine multiple shader nodes in the node editor.  The mix shader node will combine two shaders based on a given fraction.  The fac value determines the amount of the second shader in the output.

For instance the following will result in a 20% glossy (and 80% diffuse) material:


You can even use something else, such as a map or input node, to control the mix factor variably across the object.

Here I have plugged a specularity map into the factor, lighter areas on the map will receive more gloss, darker areas will receive more diffuse.


And here I have mixed in another glossy shader using an input > fresnel node as a factor, this will make grazing angles even more glossy (which is more realistic).


Note: to add a node hit Shift+A and select the node you want to use.